A power supply is an electrical device that supplies electric power to an electrical load. The primary function of a power supply is to convert electric voltage and current from a source to the correct voltage and current to power the load. As a result, power supplies are sometimes referred to as electric power converters.
A power factor (PF) of an alternating current (AC) electrical power system is defined as the ratio of a real power absorbed by a load to an apparent power flowing in a circuit, and is a dimensionless number in the closed interval of −1 to 1. A power factor of less than one indicates the voltage and current are not in phase, reducing the instantaneous product of the two. Real power is the instantaneous product of voltage and current and represents the capacity of the electricity to perform work. Apparent power is the average product of current and voltage. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power may be greater than the real power. A negative power factor occurs when the device (which is normally the load) generates power, which then flows back towards the source.
In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. Power factor correction (PFC) increases the power factor of a load, improving efficiency for the distribution system to which it is attached. Linear loads with a low power factor (such as induction motors) can be corrected with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise the power factor.
Capacitors for filtering input current noise (electromagnetic interference (EMI) filter capacitors) in PFC circuits may cause a lower PF for a switched mode power supply (SMPS). However, depending on the location of the EMI filter capacitors in a PFC circuit (e.g., after a bridge rectifier), a total harmonic distortion (THD) may increase, especially at low output loads of the SMPS. As such, a PFC technique is needed that can improve the PF while reducing the THD in the SMPS.